Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros

[Kirill A. Shutemov]

On Thu, Feb 21, 2019 at 05:16:46PM +0000, Steven Price wrote:
> >> Note that in terms of the new page walking code, these new defines are
> >> only used when walking a page table without a VMA (which isn't currently
> >> done), so architectures which don't use p?d_large currently will work
> >> fine with the generic versions. They only need to provide meaningful
> >> definitions when switching to use the walk-without-a-VMA functionality.
> > 
> > How other architectures would know that they need to provide the helpers
> > to get walk-without-a-VMA functionality? This looks very fragile to me.
> 
> Yes, you've got a good point there. This would apply to the p?d_large
> macros as well - any arch which (inadvertently) uses the generic version
> is likely to be fragile/broken.
> 
> I think probably the best option here is to scrap the generic versions
> altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which
> would enable the new functionality to those arches that opt-in. Do you
> think this would be less fragile?

These helpers are useful beyond pagewalker.

Can we actually do some grinding and make *all* archs to provide correct
helpers? Yes, it's tedious, but not that bad.

I think we could provide generic helpers for folded levels in
<asm-generic/pgtable-nop?d.h> and rest has to be provided by the arch.
Architectures that support only 2 level paging would need to provide
pgd_large(), with 3 -- pmd_large() and so on.

-- 
 Kirill A. Shutemov